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Lihua Chen, Chunming Huang, Gang Xu, Lei Miao, Jifu Shi, Jianhua Zhou, Xiudi Xiao, "Synthesis of Thermochromic W-Doped VO2 (M/R) Nanopowders by a Simple Solution-Based Process", Journal of Nanomaterials, vol. 2012, Article ID 491051, 8 pages, 2012. https://doi.org/10.1155/2012/491051
Synthesis of Thermochromic W-Doped VO2 (M/R) Nanopowders by a Simple Solution-Based Process
Thermochromic W-doped VO2 nanopowders were prepared by a novel and simple solution-based method and characterized by a variety of techniques. We mainly investigated the effect of tungsten dopant on the structural properties and phase transition of . The as-obtained nanopowders with tungsten content of ≤2.5 at% can be readily indexed as monoclinic VO2 (M) while that of 3 at% assigned into the rutile VO2 (R). The valence state of tungsten in the nanopowders is +6. TEM and XRD results show that the substitution of W atom for V in VO2 results in a decrease of the d space of the (011) plane. The phase transition temperature is determined by differential scanning calorimetry (DSC). It is found, for the first time, that the reduction of transition temperature reaches to 17 K per 1 at% of W doping with the tungsten extents of ≤1 at%, but only 9.5 K per 1 at% with the tungsten extents of >1 at%. The reason of this arises from the difficulty of the formation of - and - pairs by the increasing of W ions doping in the system.
Vanadium oxides have nearly 15–20 stable phases, meanwhile metalinsulator transition (MIT) has been reported in at least 8 vanadium oxide compounds (V2O3, VO2, V3O5, V4O7, V5O9, V6O11, V2O5, V6O13, etc.) at temperatures ranging from −147°C to 68°C [1–3], in which VO2 materials show the fully reversible phase transition between monoclinic VO2 (M) and tetragonal rutile phase VO2 (R) fascinatingly around 68°C. As a result, the resistance has a sharp change of 4–5 orders of magnitude, and the optical transmission alters correspondingly. Below the critical temperature (), VO2 is in the semiconductive state, in which the energy gap is around 0.6 eV , permitting high infrared (IR) transmission. Above , VO2 is in the metallic state, in which overlap between the Fermi level and the band eliminates the aforementioned band gap, causing vanadium dioxide to be highly reflecting or opaque in the near-infrared (NIR) region [5–8]. Furthermore, the phase transition temperature can be adjusted to near room temperature by doping, which is realized by the incorporation of metal ions into the VO2 lattice . Tungsten, molybdenum, chromium, titanium, fluorine, and niobium, and so forth are frequently used for this purpose because they produce relatively larger shifts with less dopant concentrations. It has been found that tungsten might be the most effective element [9–14]. Therefore, with such properties, VO2 materials can be considered as a promising candidate for a variety of potential applications such as energy-efficient window coatings , thermal sensors , cathode materials for reversible lithium batteries , electrical and infrared light switching device [17, 18].
So far, as an intelligent window material, the study of W-doped VO2 mainly focused on thin films and nanoparticles. It has been prepared by a variety of methods involving excimer-laser-assisted metal organic deposition (ELAMOD) , magnetron sputtering , chemical vapor deposition (CVD) , pulsed laser deposition (PLD) , and vacuum evaporation . However, all of these methods are not suitable for putting into practice because of complex control parameters, unstable technology, and the necessity of special and expensive equipment . Chemical solution deposition seems to be an alternative solution to the above problems due to its low cost and the option of metal doping. But this method usually requires specific raw materials or pretreatments which limit their practical applications . Up to now, other modified methods for synthesis of M- or R-phase vanadium dioxide have been presented such as hydrothermal processes  and reduction-hydrolysis methods . Nevertheless, long reaction time (12 h to several days) is often needed, or virulent precursor such as V2O5 is always required. Thus, more simple method for preparing vanadium dioxide with MIT property needs to be developed to promote its practical applications.
In this paper, we report a simple solution-based process to prepare pure VO2 and W-doped VO2 nanopowders with cheap and nontoxic vanadium (V) precursors and short reaction times. The characterization of the obtained nanopowders is studied through a variety of techniques. Furthermore, doping with tungsten could adjust the phase transition temperature remarkably, and thus put the thermochromic application into practice.
2. Experimental Section
2.1. Preparation of V1‒x WxO2 Nanopowders
First, a 0.5 g portion of ammonium metavanadate powders (NH4VO3, 99%, Tianjin Fuchen Chemical Reagents Factory) and appropriate amount of ammonium tungstate (N5H37W6O24·H2O, 85–90%, Sinopharm Chemical Reagent Co, Ltd.) with different W/V atom ratios were dissolved in 50 mL deionized water, respectively. Then oxalic acid dihydrate (C2H2O4·2H2O, 99.5%, Guangzhou Chemical Reagent Factory) was added to the above solution, where the molar ratio of NH4VO3 and C2H2O4·2H2O was kept at 2 : 3. The mixture was stirred continuously for 30 min to form a sky blue clear solution, which indicated that the valence of vanadium in the solution was V4+. As is known, the solution of V5+ is yellow, V4+ is blue, and that of V3+ is green, respectively. Then the above solution was dried below 100°C. Finally, W-doped VO2 products, denoting as O2 (x was appointed a delegate to the atomic ratio of W/V in the reactive precursors, , at intervals of 0.005), were obtained after annealing the collected powders at 500°C for 8 hours in nitrogen atmosphere. The possible reactions in the solution and the decomposition of the intermediate are listed as follows [27, 28]:
Powder X-ray diffraction (XRD, PANalytical B.V., X′Pert Pro MPS PW3040/60) patterns of the samples were recorded in the scanning range of 5–80° at room temperature of 25°C. The morphologies, dimensions, elemental composition, and crystallinity of the nanopowders were examined by scanning electron microscopy (SEM, Hitachi, S-4800), energy dispersive X-ray spectroscopy (EDS) attached to the SEM, transmission electron microscopy (TEM), and high-resolution TEM (JEOL, JEM-2010HR), respectively. Samples for TEM observation were prepared by dispersing in ethanol. Differential scanning calorimetry (DSC, Netzsch-Bruker, STA449F3Jupiter-TENSOR27) experiments were performed using a DuPont differential thermal analyzer under atmosphere flow in the range of 25–120°C with a heating rate of 5 K min−1, and in the measure procedure heating process alternates with cooling process. The valance state of the as-obtained O2 nanopowders was characterized by means of X-ray photoelectron spectroscopy (XPS, Thermo-V-G Scientific, ESCALAB250).
3. Results and Discussion
The morphology of the undoped and W-doped VO2 nanopowders is characterized by SEM as shown in Figure 1. It is observed in Figures 1(a) to 1(g) that the tungsten dopant concentration almost has no effect on the morphology of the nanoparticles, and the particle sizes are about 20–60 nm. The experimental results also indicate that particles will be congregated with the increase of annealing time. Especially, the particles with 2 at% W-doped are relatively uniform, and the size is about 25 nm, which is in favor of the practice application on thermochromic window coatings. As is known, small and uniform particles are relatively easy to disperse in solvent and obtain homogeneous coating. Therefore, the 2 at% W-doped sample is investigated in detail in the following experiments. EDS analysis results further confirm the existence of V, W, and O elements. The representative peaks of V and O elements appear in all of the obtained samples, and the representative peaks of W element also appear in each of W-doped products, which confirm a successful doping of W into VO2. Here we just give the EDS pattern of 2 at% W-doped sample (Figure 1(h)) as a representative example.
The XRD patterns of W-doped VO2 nanopowders with various tungsten contents are recorded in Figure 2(a). The magnified versions of the XRD data in the range of 26.5° ≤ 2θ ≤ 29° and 64° ≤ 2θ ≤ 66° are depicted in Figure 2(b) and Figure 2(c), respectively. It is found that the as-obtained samples with the tungsten extents of ≤2.5 at% can be readily indexed as monoclinic VO2 (M) (JCPDS card number 79-1655), while that of 3 at% assigned into the rutile VO2 (R) (JCPDS card number 43-1051). For the sample doped 3 at% tungsten, the peak in 26.5° ≤ 2θ ≤ 29° shifts left than the others figuring out the change of VO2 (M) (011) to VO2 (R) (110) in Figure 2(b), and meanwhile the VO2 (M) (310) peak splits into the VO2 (R) (013) and (002) in 64° ≤ 2θ ≤ 66° (Figure 2(c)). The above two phenomena together indicate the occurrence of the diagnostic feature for the structural phase transition from monoclinic to tetragonal VO2 phase, which are in good agreement with previous reports [7, 29]. Therefore, the changes in peak positions of as-obtained samples indicate that an appropriate amount of tungsten doping can promote the phase transition .
In Figure 3, XPS analysis of the as-obtained nanopowders with 2 at% W-doped is performed to understand in detail the valance state. The spectra indicate that there are four elements: oxygen, vanadium, carbon, and tungsten with binding energy peaks corresponding to C1s, O1s, V2s, V2p, V3s, W4f, W4d, and W4p in W-doped VO2 nanopowders (Figure 3(a)). The forms of carbon are possibly from surface contamination [5, 9, 30]. The data reveals that the peak at 530.3 eV is associated with O1s . The peaks located at 516.6 eV (reported values: 515.7–516.6 eV [5, 6, 9, 26, 30, 31]) and 524.0 eV (reported values: 522.6–524.0 eV [5, 6, 9, 26, 29, 30]) correspond to and (Figure 3(b)), respectively, and the binding energy of increases slightly after W doping . The peaks follow with binding energies of and at 35.28 eV and 37.45 eV, respectively. According to the standard binding energy, tungsten atoms in these nanopowders exist as W6+ (Figure 3(c)) . N-type semiconductor could form as W6+ ions replace V4+ ions.
TEM images of the undoped VO2 and 2 at% W-doped VO2 nanopowders are shown in Figures 4(a) and 4(c). The morphologies and sizes of the as-obtained samples are consistent with those of SEM images in Figures 1(a) and 1(e). Figures 4(b) and 4(d) show the lattice-resolved HRTEM images. The fringe spacing is 0.321 nm for undoped VO2 (Figure 4(b)) sample, which is consistent with the space of the (011) plane of monoclinic VO2 (M) phase [32, 33], and the fringe spacing reduces to 0.314 nm (Figure 4(d)) for the sample of 2 at% W-doped VO2. This decreased tendency of the fringe spacing with W doping is consistent with the calculated results by Scherrer formula. As the radius of W6+ ion (60 pm ) is smaller than that of the V4+ ion about 63 pm. The interstitial W6+ ions will cause the atoms to have larger interatomic spacings, and the interatomic spacings will be reduced in the case of substitutional defects with W6+ ions. The TEM results suggest that the substitution of W6+ ions for V4+ plays a dominant role in this work, which results in the reduction of spacing. As the tungsten dopant concentration is 2 at%, the (011) peak of monoclinic VO2 (M) shifts from 27.74° (undoped VO2) to 27.79°, indicating the decrease of the crystal lattice spacing according to the Bragg equation (; ) [29, 35, 36]. As regards the rule of substitution or interstitial of W6+ ions for V4+ is unknown, and it needs further research.
When the phase transition of VO2 occurs, it exhibits a noticeable endothermal or exothermal profile in the DSC curve. Figure 5(a) shows the typical DSC curves of undoped and 2 at% W-doped VO2 nanopowders. With 2 at% W-doped sample, Mott phase transition arises at around 44°C and 34.5°C for the heating and cooling cycles, compared to 71°C and 58°C for the undoped VO2, respectively. The phase transition can be modified under the different factors such as defect density or lattice change [3, 29]. The appearance of endothermal and exothermal peaks during the heating and cooling process confirms the first-order transition between monoclinic VO2 (M) and tetragonal rutile VO2 (R) . To be vital for the practical thermochromic effect applications, the phase transition temperature of W-doped must be approaching to room temperature. In this case, the phase transition temperature could be reduced to 35°C with 3 at% W-doped in Figure 5(b).
A nonlinear decrease of the phase transition temperature with increasing percent of tungsten atom incorporation is observed upon heating process (Figure 5(c)). And the nonlinear decrease can be described by two linear fits. The reduction of transition temperature is estimated to be about 17 K per 1 at% of W doping with the tungsten extents of ≤1 at%, but only 9.5 K per 1 at% with the tungsten extents of >1 at%. With tungsten ion doping into VO2, the reaction takes place as follows: , which results in the formation of V3+-V4+ and V3+-W6+ pairs . Then the transition temperature will be reduced due to the loss of direct bonding between V ions, which is resulted from the forming of the pairs. We can now assume that the change of transition temperature is determined by the difficulty of initial formation of V3+-V4+ and V3+-W6+ pairs. At the beginning, the pairs form easily with lower tungsten dopant concentration, so the transition temperature could remarkably change. By following the increase of W ions, it becomes relatively difficult to form the V3+-V4+ and V3+-W6+ pairs right away, resulting in a more gradual change in the transition temperature.
Well-crystallized nanopowders of W-doped VO2 (M/R) were successfully synthesized by a simple solution-based process through the reaction of ammonium metavanadate (NH4VO3) and oxalic acid dihydrate (C2H2O4·2H2O) followed by adding to appropriate ammonium tungstate (N5H37W6O24·H2O). It is shown that tungsten dopant concentration almost has no effect on the morphology of the nanoparticles and the granular particles are about 20–60 nm. As-obtained nanopowders with the tungsten extents of ≤2.5 at% can be readily indexed as monoclinic VO2 (M), while that of 3 at% assigned into the rutile VO2 (R). Substitutional W6+ ions could reduce the interatomic spacings, which results in the decrease of the space of the (011) plane in monoclinic VO2 (M) phase. Moreover, we found that the difficulty level in initial formation of V3+-V4+ and V3+-W6+ pairs determines the rate of change of the critical temperature. The reduction of transition temperature is estimated to be about 17 K per 1 at% of W doping with the tungsten extents of ≤1 at%, only about 9.5 K per 1 at% with the tungsten extents of >1 at%. With 3 at% W-doped VO2, the phase transition temperature can be reduced to 35°C. In short, this paper provides a simple solution-based method to prepare W-doped VO2 nanopowders with good thermochromic properties showing the transition temperature required to building glazing, which is in favor of promoting the practical applications of smart window.
This work is supported by the National Natural Science Foundation of China (no. 51102235) and National Science Foundation of Guangdong Province (no. 9451007006004079).
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